Three groups comprised the bulk of the uninsured in 2010, including foreign-born residents who are not U.S. citizens, young adults ages 19 to 25 and low-income families with an annual household income of less than $25,000.

Much of the declines in insured rates in recent years can be attributed to the loss of employer-provided coverage, which fell amid sustained unemployment and as employers continued to cut back on benefits.

The percentage of people who had health insurance through their employers fell to 55.3% in 2010 from 56.1% the year before, continuing a long, downward trend. In 2000, 64.1% of the population received health insurance through their employers.

"As the job market remains weak, Americans can no longer depend on their workplace for consistent affordable coverage," said Elise Gould, Director of Health Policy Research for the Economic Policy Institute, a Washington-based think tank.

Some employers have stopped offering health insurance, while others are passing along more of the cost to their employees, said Gould. As a result, some workers are abandoning their employer's plans because the premiums have become too expensive, she said.

The average health insurance premium for family coverage has more than doubled over the past decade to $13,770 a year, according to the Kaiser Family Foundation, a non-profit which focuses on health care policy and issues.

With fewer Americans receiving health care coverage through their employers, government-funded programs like Medicare, Medicaid, military health care, the Children's Health Care Program (CHIPS) and coverage offered by various states have had to pick up the slack.

In 2010, 31% of Americans relied on the government for health insurance, up from 24.2% in 1999.

Many of the new government beneficiaries are children, according to Gould. Still, Census reported that 9.8% of children under age 18 are uninsured despite the government programs targeting them like CHIPS and Medicaid, which is also open to their parents.

Adults without dependent children, however, are not eligible for Medicaid in most states under federal rules, said Rachel Garfield, a senior researcher on the Kaiser Commission on Medicaid and the Uninsured. It's this group that accounts for a large portion of the increase in the uninsured.

Nearly one-in-four working-age adults are uninsured, said Gould. She said it won't be until 2014, when Obama's Affordable Care Act fully kicks in, that more people will be able to find affordable health care coverage.

Kathleen Sebelius, the Secretary of Health and Human Services, said the report is evidence that the Obama administration's health care reform is already starting to work.

Citing a provision that went into effect last year that allows parents to keep their children on their health insurance policy until they are 26 years old, Sebelius noted that the percentage of young adults ages 18 to 24 who were insured increased to 72.8% in 2010 from 70.7% in 2009.

"We expect even more will gain coverage in 2011 when the policy is fully phased in," wrote Sebelius on HealthCare.gov, a web site run by the federal government.

Among the states, Texas had the highest percentage of uninsured; at 24.6%. Still, it was an improvement from the 26.1% who were uninsured in 2009. New Mexico had the second highest rate of uninsured people with 21.6%. Nevada was third at 21.3% and Mississippi's rate jumped to 21.1% from 17.3% in 2009.

Despite being home to a high percentage of retirees, many of whom are covered by Medicare, Florida had an uninsured rate of 20.8% down from 21.7% in 2009.

In Massachusetts, where former governor Mitt Romney instituted his controversial state-wide health insurance plan in 2006, only 5.6% of the population lacks coverage, the lowest rate of uninsured of any state.

Other states that boasted a smaller percentage of uninsured included Hawaii (7.7%), Wisconsin and Maine (both 9.4%) and Vermont (9.5%).